You get an NHS number when you register with a GP - it's literally just the number they use to identify you in their system. You just pick up a registration form from the GP, fill it out, hand it in and you'll get your NHS number in the mail about a week later. You will need proof of UK address though (to show you live within the GP catchment area) so I'd get your name on something ASAP when you arrive.
If you need to make an appointment you normally just have to wait a week or so for them to put you in the system and then you can make as many appointments as you need to.
However, if you want to see an OB, you do it though your GP anyway. You don't 'find an OB' as such... everything OB-related is usually done through your GP's office - if you need to see an OB specifically, normally you need to get a referral to one from your GP first.
See here for information about pre-natal care:
http://www.nhs.uk/conditions/pregnancy-and-baby/pages/antenatal-midwife-care-pregnant.aspxSent from my iPhone using Tapatalk